The present invention relates to a sheet material web and an associated web feeding and handling apparatus which feeds the web in a feed direction longitudinally of itself and performs a work operation on the web. The invention deals more particularly with such an apparatus having a friction drive for feeding the web through the apparatus, and alignment indicia on the web which the apparatus reads as the web is fed to maintain the web in proper alignment with respect to the feed direction.
The present invention may be used with various types of apparatus for performing a work operation on a web of sheet material such as a plotter, a recorder, or a signmaking apparatus. However, for the purposes of clarity and simplicity, the invention will be disclosed in connection with the description of a signmaking apparatus.
Typically, information describing a desired sign graphic, such as an alphanumeric inscription or a geometric shape, is programmed into the signmaking apparatus to cause it to automatically cut the graphic from a continuous length of signmaking web stock. The web stock is stored as a roll and is drawn by a pair of feed sprockets having pins which cooperate with rows of holes punched or otherwise formed in two side edge portions of the web. The sprocket drive not only serves to feed the web through the signmaking apparatus, but also provides a means for maintaining the web in proper alignment with respect to the feed direction as the web is fed and worked on by the apparatus. That is, the sprocket drive prevents skewing of the web to either the right or left of the feed direction as the web is fed through the device.
The signmaking apparatus described above, as well as other web working apparatus, may handle a variety of types of webs and may operate with various types of instruments. For example, the signmaking apparatus is often used to plot a sign pattern on a paper web before the relatively expensive signmaking stock is cut. This being the case, it would be advantageous to have a web and an associated web handling apparatus that did not require a sprocket drive and a corresponding pattern of sprocket holes punched in the web for feeding the web and maintaining it in alignment during the work operation. Elimination of the sprocket drive and the hole pattern in the web would not only reduce the cost of both the apparatus and the web, but also would allow for a simple and rapid interchange of the various types of webs used with the apparatus.
One known alternative to a sprocket drive is a drive whereby a sheet of work material is fed through a nip formed by at least two roller elements, one of which is driven. The pressure exerted on the work material by the roller elements, acts to pull the work material through the nip. Drives of this type are referred to by those skilled in the art as, friction drives.
In order for a piece of sheet material to be fed through a friction drive and maintain its alignment, the diameter of the rolling elements must be identical along the width of the sheet material. This has proven to be difficult because slight variations in the diameter of the rolling elements results in the work material becoming skewed relative to the direction in which it is being fed. In addition, the rolling elements in a friction drive have the tendency to skid on the work material, thereby further exacerbating the difficulties described above.
Historically, the mechanism by which manufacturers of friction drives have attempted to overcome the deleterious effects of material misalignment has been the grit wheel. Grit wheels generally take the form of cylinders sequentially spaced along the length of a roller. Each grit wheel has an anti-skid treatment on its outer periphery. In order to ensure that the work material feeds through the apparatus without becoming skewed, the outer diameters of all of the grit wheels used on a particular roller must be identical, as such they can become quite expensive. Moreover, since the grit wheels are manufactured as matched sets, if one grit wheel wears faster than another, all of them must be reworked or replaced.
In addition to the foregoing, it should also be appreciated that as the cutter or plotting pen of the signmaker works on the web, it can create a drag in the center portion or work area of the web. Therefore, the web tends to bow in the lateral direction as it is fed through the apparatus and the work operation is performed. Even a sprocket drive which effectively maintains the web in alignment with the feed direction, does little to correct the lateral bowing of the web that occurs during the work operation.
It is therefore an object of the present invention to provide a sheet material web and an associated web handling apparatus which do not utilize a sprocket drive and a corresponding pattern of sprocket holes formed in the web to feed the web through the apparatus and maintain proper web alignment.
It is a further object of the present invention to provide such a web and associated handling device, wherein lateral bowing of the web during the work operation is either compensated for or eliminated.
It is still another object of the present invention to provide a web handling device which can actively compensate for web misalignment.